(1) Field of the Invention
The present invention generally relates to turbine rotors, such as those used in steam turbines, gas turbine engines, and jet engines. More particularly, this invention relates to a rotor and method of producing a monolithic rotor containing two or more alloys within separate regions of the rotor resulting in a transition zone between different alloy regions, and to a method of determining the shape of the transition zone to enable the final machined geometry of the rotor to reduce any thermal instability attributable to the presence of the transition zone.
(2) Description of the Related Art
Rotors used in steam turbines, gas turbines and jet engines typically experience a range of operating conditions along their lengths. The different operating conditions complicate the selection of a suitable rotor material and the manufacturing of the rotor because a material optimized to satisfy one operating condition may not be optimal for meeting another operating condition. For instance, the inlet and exhaust areas of a steam turbine rotor have different material property requirements. The high temperature inlet region typically requires a material with high creep rupture strength but only moderate toughness. The exhaust area, on the other hand, does not demand the same level of high temperature creep strength, but suitable materials typically must have very high toughness because of the high loads imposed by long turbine blades used in the exhaust area.
Because a monolithic rotor (i.e., a rotor that is not an assembly) of a single chemistry cannot meet the property requirements of each of the LP, IP and HP stages for the reasons discussed above, rotors constructed by assembling segments of different chemistries are widely used. For example, large steam turbines typically have a bolted construction made up of separate rotors contained in separate shells or hoods for use in different sections of the turbine. More recently, the steam turbine industry has favored CrMoV low alloy steels for use in the HP stage and NiCrMoV for use in the LP stage, though NiMoV low alloy steels have also been widely used as materials for the various stages. Smaller steam turbines may make use of a mid-span coupling to bolt high and low temperature components together within one shell. Finally, rotors for gas turbines and jet engines are often constructed by bolting a series of disks and shafts together. While rotors having a bolted construction are widely used, they suffer from several disadvantages including increased numbers of parts, increased assembly requirements, increased length of the rotor assembly, and more balance complexity.
Another method of combining different materials in a single rotor is to weld together rotor segments formed of dissimilar materials, forming what may be termed a multiple alloy rotor (MAR). However, a welded rotor construction also has disadvantages, such as high investment costs for the welding equipment, additional production costs for weld preparation and welding, long production times to produce, inspect and upgrade the weld, and increased cost and production time caused by the need for post weld heat treatment. The strength of rotors having a welded construction can also be limited due to a need to maintain a low carbon content in the weld, and the propensity for high numbers of small non-metallic inclusions that reduce load carrying capability.
The capability of producing a monolithic MAR would address the above-noted shortcomings of assembled MAR's. Furthermore, monolithic MAR's would be particularly well suited for meeting the demand for higher efficiency steam turbines whose requirements include low pressure (LP), intermediate pressure (IP) and high pressure (HP) stages (or combinations thereof) with additional stages in areas normally occupied by couplings. Consumable electrode remelting techniques such as electro-slag remelting (ESR) and vacuum arc remelting (VAR) methods offer flexibility for producing components that contain alloy combinations, and therefore has been considered for producing monolithic MAR's. As an example, U.S. Pat. No. 6,350,325 to Ewald et al. discloses an ESR method of producing a dual alloy rotor from 12Cr-type alloys that have different levels of alloying constituents, but are sufficiently close in composition so as to have substantially identical austenitizing temperatures. Ewald et al. also disclose that, because the alloys have similar compositions, problems can be avoided that are associated with mixing of alloys having significantly different material properties, which results in the formation of a transition zone (TZ) between regions of the rotor formed by the different alloys.
One such problem is thermal stability arising from the massive size of a rotating rotor supported by bearings at each end of the rotor. When supported in this manner, a rotor behaves as a simply supported beam structure and will deflect in reaction to the centrifugal load always present at operational conditions, with the largest deflection being near the center of the rotor. Because of the inherent asymmetry of the transition zone within a MAR rotor, deflection significantly increases when the rotor is at its elevated operating temperatures. As the rotor rotates about its bent centerline, the rotor material is subjected to high cycle fatigue as a result of being in tension and then in compression with each rotation. Consequently, reducing deflection by minimizing material asymmetry is necessary to maximize the life of a MAR rotor and the turbine in which it is installed. One solution is to limit the rotor to alloys with similar compositions. However, this restriction limits the ability to optimize the compositions of the LP, IP and HP rotor sections for their operating environments and cost. For example, such a limitation has dissuaded the manufacture of a monolithic MAR whose HP stage is formed of CrMoV and its LP stage is formed of NiCrMoV. Therefore, it would be desirable if an improved process were available for producing turbine rotors of different alloy compositions.